This invention relates to multielement electro-optic imaging systems and, more particularly, to techniques for compensating such systems for the non-uniformities introduced by the individual detector elements.
In a scanned imaging system, an image sensitive detector element is scanned across a scene and the response of the element is read out and utilized to reconstitute an image of the scene. A pure scanned system, however, is inherently limited in its ultimate performance, a characteristic which prompted the development of improving imaging techniques, such as the two dimensional focal plane array.
In the two dimensional focal plane, the desired scene is optically focused on a matrix of multiple detector elements so that each element images a particular portion of the scene. Such focal planes exhibit significant advantages in a number of important applications, such as in the field of infrared imaging. In this area, the employment of infrared focal planes has become particularly desirable as a result of the development of improved signal processing techniques which allow the detectors to be integrated with the necessary electronic signal conditioning for the focal plane. By interfacing detectors with signal processing electronics in a high density configuration using photolithographic processes, a potentially low cost focal plane can be achieved. The integrated focal plane can also exhibit enhanced performance as a result of the larger number of detectors which may be included in a focal plane of a given size.
These integrated techniques have made feasible high density non-scanned, or staring, infrared systems. Non-scanned thermal imagers are advantageous in many applications because of their characteristic low weight, simple optics, immunity to shock, and potentially low cost. In addition, a staring system is inherently more sensitive than a scanned system because of the longer integration times which are achievable with staring sensors.
Although multielement sensing systems are thus potentially a very advantageous approach to imaging, the multielement nature of such systems introduces a limitation on system performance, due to the lack of uniformity among the individual elements. This restriction is particularly important in the infrared region, where the inherently poor radiation contrast in that portion of the spectrum imposes severe demands on element-to-element signal uniformity. Thus, the responsivity and DC offset variations in a typical infrared detector array give rise to fixed pattern noise of a magnitude which can readily mask low contrast thermal images.
In the past, offset non-uniformities, which result from variations among the detector elements of an array in leakage current, bias, and responsivity to the background, have ordinarily been corrected by means of a uniform temperature reference. A correction signal is obtained in this technique by recording the individual response of each detector element to the infrared radiation which emanates from the uniform temperature reference. The correction is accomplished on the input unit cell by subtraction off the focal plane in a signal processing area where either analog or digital scaling may be applied. In a staring focal plane, however, it is often inconvenient to provide such a temperature reference. In addition, such references, and their accompanying shuttering systems, add additional cost and complications to an imaging system. Such temperature references may also require calibration before an imaging system can be rendered operational. If it is also necessary to correct for AC or gain non-uniformities, a second temperature reference at a different temperature must also be provided to accomplish the gain correction, which further increases the complexity of the system. The off focal plane signal processing then performs the correction in the input unit cell both by subtraction and by variable gain input circuits, utilizing the individual responses of the detectors to each of the uniform temperature scenes.
Therefore, a need has developed in the art for an improved technique to compensate for non-uniformities in a multielement detector array.